Nitroxide radicals are being investigated for many biological and biomedical applications, such as spin probes, antioxidants, radiation protectors, electron paramagnetic resonance imaging (EPRI), and magnetic resonance imaging (MRI). These applications rely mainly on the use of nitroxide monoradicals with the total spin quantum number (S) of 1/2. This R21 project will explore a new class of radicals, high-spin nitroxide diradicals with S = 1, for biomedical imaging applications. Because the signal intensity in MRI scales with the factor of S(S + 1) of paramagnetic agent, we expect that significant increases in sensitivity could be attained using high-spin S >1/2 radicals as contrast agents. The specific hypothesis behind this proposal is that the S = 1 diradicals, especially the rigid scaffolds of S = 1 diradicals, can be made to possess adequate relaxivity and redox properties for functional MRI contrast agents. Diradicals and scaffolds of diradicals with rigid structure will be prepared by organic synthesis and their selected properties affecting 1H water relaxivity will be studied. Values of S(S + 1) will be measured by superconducting quantum interference device (SQUID) magnetometry and EPR spectroscopy. Rotational correlation times and electron spin relaxation times (T1 and T2) will be measured by EPR spectroscopy. Stability in the presence of ascorbate will be determined. Nitroxides with most promising relaxivities and redox properties will be subjected to an in vivo study. This information will provide an assessment of the key factors affecting the 1H water relaxivity of the nitroxide radicals, a foundation for our long term goal of the development of highly sensitive, functional, and safe MRI contrast agents based upon stable, redox-active high-spin nitroxide polyradicals with large values of S. Development of contrast agents based on organic compounds with strong paramagnetic properties will expand the frontier of biomedical imaging research. Various structural modifications of organic compounds for specific targeting and property tuning (magnetism, redox, solubility, toxicity) can be efficiently implemented by organic synthesis. Organic-based contrast agents will provide an alternative to metal-based agents, in which the property tuning is limited and their applications have been associated with the risk of exposure to highly toxic metal ions. Such organic-based agents should render safe, effective tools for early detection and diagnosis of diseases and for modern approaches to drug discovery. PUBLIC HEALTH RELEVANCE: Contrast agents based on organic compounds with strong paramagnetic properties provide alternative MRI contrast media to the metal-based agents, avoiding the risk factors associated with the release of highly toxic metal ions. This project will explore the feasibility of sensitive and safe contrast agents based on organic radicals. Such agents may ultimately render effective tools for early detection and diagnosis of diseases, and for modern approaches to drug discovery.